1. Field of the Invention
The present invention relates to an optical transmitter having a laser diode and capable of controlling its driving current by an automatic power control (automatic power control: APC hererinafter) circuit, and more particularly to an optical transmitter capable of controlling the optical output in transient response at the time of optical output cut-off or cancellation of optical output cut-off.
2. Prior Art
An optical transmitter includes a laser diode (light amplification by stimulated emission of radiation diode: LD hereinafter), and by controlling its driving current, a signal corresponding to transmission information is set to an optical receiver. However, since the LD changes in its characteristics depending on temperature changes or aging effects, if the same driving current is supplied, the output optical power from the LD fluctuates. As a result, the transmission output power may decrease to deteriorate the S/N (signal to noise) ratio, or the LD or the optical receiving element at the transmission destination may be broken by an excessive driving current. In the conventional optical transmitter, accordingly, to enhance its reliability, by using APC circuit and driving current control circuit, it is attempted to compensate for fluctuations of characteristics due to temperature changes or aging effects of LD and limit excessive driving current.
The APC circuit receives part of the LD output or back light by a photo diode, feeds back its reception power to control the driving current of the LD, so that a desired optical power is constantly delivered from the LD. The driving current control circuit controls the driving current supplied to the LD so as not to exceeds a limit value predetermined according to the LD standard.
FIG. 1 shows an outline of such conventionally proposed constitution of an optical transmitter. This optical transmitter comprises an LD 11 for delivering a signal light 10 depending on a driving current, and a photo diode 13 for receiving part of output light of the LD 11 or back light 12. The optical transmitter further comprises a current-voltage converting circuit 14 for converting a received current generated by receiving part of the output light from the LD 11 or back light 12 by the photo diode 13 into a voltage value, an average detecting circuit 15 for detecting the average of voltage values converted by the current-voltage converting circuit 14, and a reference current control circuit 16 for detecting the mark rate which is the appearance ratio of xe2x80x9c1xe2x80x9d state and xe2x80x9c0xe2x80x9d state of transmission data, and issuing a corresponding voltage value, in which an average voltage V1 detected by the average detecting circuit 15 and a reference voltage V2 detected by the reference voltage control circuit 16 depending on the mark rate of transmission rate are entered into an optical output control circuit 17.
The optical output control circuit 17 issues a control signal so that the entered average voltage V1 and reference voltage V2 may be equal to each other, and its output is entered into a driving current control circuit 18. The driving current control circuit 18 can control a laser driving circuit 19 for generating a driving current of the LD 11 as disclosed in, for example, Japanese Laid-open Patent No. 7-193540, and by controlling so that the laser driving circuit 19 may not pass any current exceeding the preset value of the LD 11, the LD 11 is prevented from generating an excessive optical output. The output of the driving current control circuit 18 which is controlled thus by the optical output control circuit 17 is entered in the laser driving circuit 19, and the driving current of the LD 11 can be controlled, so that the average voltage value detected by the average detecting circuit 15 by changing the optical power of the optical signal can be controlled.
That is, the optical output control circuit 17 can raise the average voltage by controlling the driving current control circuit 18 and laser driving circuit 19 so that the power of the output light from the LD 11 may be larger when the entered average voltage V1 is smaller than the reference voltage V2, and thereby the difference between the average voltage V1 and reference voltage V2 can be decreased. On the other hand, when the entered average voltage V1 is larger than the reference voltage V2, by controlling the driving current control circuit 18 and the laser driving circuit 19 so that the power of the output light from the LD 11 may be smaller, the average voltage is lowered, and the difference between the average voltage V1 and reference voltage V2 can be decreased. In this way, the optical output control circuit 17 issues a control signal corresponding to the difference between the average voltage V1 and reference voltage V2 to the driving current control circuit 18 in order to obtain a desired output light from the LD 11. As a result, the optical output power of the LD 11 is maintained at a constant optical output depending on the reference voltage V2 of the reference voltage control circuit 16.
In such optical transmitter, it is possible to change the driving current by the laser driving circuit 19 so as to compensate for threshold current due to, for example, temperature changes of the LD 11 or fluctuations of the output power of the LD 11 due to variation of slope efficiency. Or, against characteristic changes due to aging effects of the LD 11, the driving current can be changed so as to compensate for the variation, so that the output power of the LD 11 may be maintained constant.
By the way, the optical transmitter includes an optical output cut-off processing circuit 22 for controlling the laser driving circuit 19 for cutting off the optical output by an optical output forced cut-off input 20 from outside and a detection signal 21 of the preset condition of optical output cut-off. Accordingly, the optical output during optical output constant control can be cut off. Depending on the preset condition of optical output cut-off, herein, the optical output cut-off may be detected when the aforesaid mark rate is lower than the specified value, for example, due to cut-off the transmission data or clock. Thus, when the optical output cut-off is recognized by the optical output forced cut-off input 20 or optical output cut-off condition detection signal 21, the optical output cut-off processing circuit 22 controls the laser driving circuit 19, and cuts off the optical output from the LD 11. On the other hand, when the optical output forced cut-off input 20 or optical output cut-off condition detection signal 21 is canceled, the optical output cut-off control of the laser driving circuit 19 is canceled, so that an optical signal controlled at a constant optical output can be issued.
The technology relating to such optical transmitter is disclosed, for example, in Japanese Laid-open Patent No. 7-193540 and Japanese Laid-open Patent No. 5-075547.
Japanese Laid-open Patent No. 7-193540 discloses an optical transmitter for communication for transmitting video by controlling the optical output constant by an optical automatic output control (auto power control) circuit, and using a semiconductor laser diode controlled so that the driving current may not exceed a specific value by using a limiter circuit, in which for the safety of the operator in case of emergency, the circuit for output shut-down of the semiconductor laser diode is realized simply by using a limiter circuit.
Japanese Laid-open Patent No. 5-075547 discloses an optical transmitter comprising a semiconductor laser, a semiconductor laser driving circuit for feeding a semiconductor laser driving current composed of a light modulation signal current for modulating the intensity of exit light of the semiconductor laser by an input signal and a direct-current bias signal into the semiconductor laser, a mark rate detecting circuit for detecting the integral value of pulse component of input signal, and a shut-off circuit for turning on or off the supply of the semiconductor laser driving current. According to a function provided in the optical transmitter, the input signal is branched into two, one is put into the semiconductor laser driving circuit, and other is put into the mark rate detecting circuit, and when the output of the mark rate detecting circuit becomes smaller than a real number specified value A or larger than real number specified value B satisfying the relation of A less than B, the shut-off circuit is operated, and emission of the semiconductor laser is prohibited. The optical transmitter disclosed in these publications is basically same as shown in FIG. 1.
However, including the optical transmitters disclosed in these publications, the conventional optical transmitter shown in FIG. 1 is known to have a problem of generation of optical output power exceeding the standard of the LD 11 in transient response at the time of optical output cut-off or cancellation of optical output cut-off by the optical output forced cut-off input 20 or optical output cut-off condition detection signal 21. The conventional optical transmitter has the driving current control circuit 18, and prevents flow of excessive driving current into the LD 11, but it cannot prevent excessive output of the LD 11 at the time of transient response.
FIG. 2 shows an outline of the relation between driving current and optical output from the LD. In the LD, as shown herein, the optical output increases, depending on the operating temperature, after a certain driving current value. In the optical transmitter using the LD, it is required to set the driving current value so that a desired optical output may be obtained from the LD whether the operating temperature is high or low. For example, supposing that the driving current limit value is set at I0, the LD output power at low temperature is appropriate, but the LD output at high temperature is limited, and a desired set output power cannot be obtained. On the other hand, supposing that the driving current limit value is set at I1, the LD output power at high temperature is appropriate, but the LD output at low temperature generates a high optical output exceeding the optical output standard of the LD. Usually, the driving current limit set value Is set so that a desired optical output may be obtained even if the LD is at high temperature.
By contrast, composing a reference by using parts having temperature characteristics, in other method, it may be considered to change the driving current limit value by referring to the reference changing similarly depending on the operating temperature. It, however, requires adjustment depending on the characteristics of individual LDs, and a same adjustment is needed for the driving current value for preventing the excessive output at the time of transient response, which results in a higher cost.
A further detail is described below about generation of excessive output at the time of transient response in such optical transmitter having the driving current limit set at Is. As known as a cause of generation of excessive output at the time of transient response by the optical transmitter, the output average voltage V1 from the average detecting circuit 15, the output reference voltage V2 from the reference voltage control circuit 16, and the output from the driving current control circuit 18 have individually different time constants, and each time constant varies in transient response due to optical output cut-off or its cancellation.
FIG. 3 shows the operation of transient response at the time of cancellation of optical output cut-off in the optical transmitter shown in FIG. 1 for explaining such problem. Suppose, hereinafter, that the average voltage V1 changes at rise time tr1 and fall time tf1 by the time constant of the average detecting circuit 15, and that the reference voltage V2 changes at rise time tr2 and fall time tf2 by the time constant of the reference voltage control circuit 16. Also suppose that the control of optical output cut-off is started at time T0 by the optical output forced cut-off input 20 or optical output cut-off condition detection signal 21, and that the control of the optical output cut-off is canceled at time T1 (FIG. 3(a) 23).
When the control of optical output cut-off is started at time T0, since the average detecting circuit 15 and reference voltage control circuit 16 individually have the time constant depending on their load or operating conditions, the average voltage V1 and reference voltage V2 cannot be changed at the same timing as the timing of optical output cut-off. The average voltage V1 delivered from the average detecting circuit 15 by way of the photo diode 13 and voltage-current converting circuit 14 changes at the timing of rise time tr1 or fall time tf1 as shown in FIG. 3(b) by its intrinsic time constant (FIG. 3(b) 24). Herein, the reference voltage V2 delivered from the reference voltage control circuit 16 is directly a voltage depending on the mark rate of the detected transmission data regardless of the optical output cut-off (FIG.3 (b) 25).
The optical output control circuit 17 controls the driving current of the LD 11 so that the average voltage V1 and reference voltage V2 may be equal to each other as far as the average detection circuit 15 is an ideal circuit not having time constant, and therefore can control the cancellation of optical output cut-off nearly at the same timing depending on the cancellation of the optical output cut-off input 23 (FIG. 3(c) 26). However, since the average voltage V1 changes by a time constant, the optical output control circuit 17 produces an output so that V1 and V2 may be equal to each other, so as to, for example, raise the optical output (FIG. 3(c) 27).
The optical output control circuit 17 can cancel the output of the driving current control circuit 18 at the same timing as when canceling the optical output cut-off input 23 as far as the average detection circuit 15 is an ideal circuit not having time constant (FIG. 3(d) 28). However, since it always has a certain time constant, it cannot control the optical output cut-off by lowering the driving current nearly at the same timing as the optical output cut-off input 23. Accordingly, the driving current control circuit 18 operates to deliver a driving current depending on the output of the optical output control circuit 17 shown in FIG. 3(c) to the laser driving circuit 19 (FIG. 3(d) 29). In the laser driving circuit 19, therefore, although the driving current to be fed into the LD 11 does not reach the driving current limit set value Is shown in FIG. 2 (FIG. 3(d) 30), a driving current exceeding the maximum rated output 31 of the LD 11 is to be fed, and the driving current is controlled so as to coincide with the predetermined driving current limit set value Is. That is, as shown in FIG. 2, although it is within the optical output limit 30 by the driving current control circuit 8 determined as the driving current limit set value Is being intrinsic to the LD, an excessive output light exceeding the LD maximum rated output value 31 is generated. Moreover, when canceling the optical output cut-off, as shown in FIG. 3(d), the laser driving circuit 19 is controlled so as to supplying a driving current exceeding the maximum rated output.
Thus, the optical power issued from the LD 11 can be generated an optical output power within the standard generated by the output 28 of the driving current control circuit 18 in an ideal environment not having time constant (FIG. 3(e) 32). However, by having the time constants, when the optical output control circuit 17 controls to raise the optical output, the LD 11 generates an optical output power exceeds the standard (FIG. 3(e) 33).
Next is explained the relation between the average voltage V1 and reference voltage V2 to be entered in the optical output control circuit 17 at the time of transient response in optical output cut-off or cancellation of optical output cut-off.
FIG. 4 shows the transient response changes of optical output in the case of change of V1 and V2 by the same value as the average detecting circuit 15 and reference voltage control circuit 16 change by the same time constant. The output of the current-voltage converting circuit 14 converts the monitored current into a voltage by the resistance. In FIG. 4, a resistance is connected to the supply voltage as a method of converting from current into voltage, and the output voltage of the circuit is a supply voltage at the time of optical output cut-off, and a voltage lower than the supply voltage by the portion of voltage drop due to resistance at the time of optical output. In this way, when the optical output cut-off is started at time T0 and the optical output cut-off is canceled at time T1, the optical output of the LD 11 driven by the laser driving circuit 19 gradually stops at a certain time constant as the average voltage V1 and reference voltage V2 change at the same timing, and the normal voltage is restored gradually at the time of cancellation of optical output cut-off.
FIG. 5 shows transient response changes of optical output when the average voltage V1 is larger than the reference voltage V2 due to change at different time constants. As show in FIG. 5(a), when the optical output cut-off is started at time T0 and the optical output cut-off is canceled at time T1, the average voltage V1 and reference voltage V2 are entered in the optical output control circuit 17 as shown in FIG. 5(b). As a result, a state of average voltage V1 greater than reference voltage V2 occurs at the time of transient response by cancellation of optical output cut-off, and generation of such excessive optical output may not be suppressed (FIG. 5(c) 35). Herein, in the case of average voltage V1 greater than reference voltage V2, since the monitor current is smaller than a desired output (the average voltage V1 is a voltage due to voltage drop by the resistance connected to the power source, and hence when the average voltage V1 is high, the motor current is small), the optical output control circuit 17 judges that the optical output is lowered, and hence controls to raise the optical output.
FIG. 6 shows transient response when the average voltage V1 is not larger than the reference voltage V2 due to change at different time constants. As shown in FIG. 6(a), when the optical output cut-off is started at time T0 and the optical output cut-off is canceled at time T1, the average voltage V1 and reference voltage V2 are entered in the optical output control circuit 17 as shown in FIG. 6(b). As a result, since the state of average voltage V1 less than reference voltage V2 is maintained also at the time of transient response due to cancellation of optical output cut-off, if the output of the average detecting circuit 15 is lower than the reference voltage V2 which is one of the inputs to the optical output control circuit 17 (that is, the monitor current is larger), it is judged that an output larger than a desired output is produced, so that excessive optical output is not generated (FIG. 6(c) 35).
As mentioned herein, as the output of the reference voltage control circuit 16 of the optical output control circuit 17, the output of the average detecting circuit 15, and the driving current control circuit 18 change by different time constants by the input of optical output cut-off, the LD may produce a problem that an output exceeds the standard optical power at the time of transient response. As a result, the LD or the receiving side element is broken or deteriorates, and the reliability is extremely lowered. However, by keeping the relation of average voltage V1 less than reference voltage V2 between the average voltage V1 and reference voltage V2 entered in the optical output control circuit 17 all the time, generation of excessive optical output can be suppressed at the time of transient response. Usually, since these time constants depend on the constituent parts, it is hard to adjust in the manufacturing stage.
On the other hand, in the technology relating to the optical transmitter disclosed in Japanese Laid-open Patent No. 7-193540 or Japanese Laid-open Patent No. 5-075547, nothing is considered about excessive output at the time of transient response by optical output cut-off or its cancellation, such problems are inevitable.
It is an object of the invention to present an optical transmitter having an APC circuit capable of detecting the output light within the standard from the LD even at the time of transient response due to optical output cut-off or cancellation of optical output cut-off.
The invention relates to an optical transmitter which comprises (a) optical transmitting unit for issuing a signal light, (b) optical receiving unit for receiving part of the output light from the optical transmitting unit, (c) transmission level detector for detecting a transmission level depending on the reception power by the optical receiving unit, (d) reference level generator for generating a reference level on the basis of a mark rate of transmission data, (e) driving current changing unit for changing the driving current so that the transmission level detected by the transmission level detector may be equal to the reference level generated by the reference level generator, (f) driving current feed unit for changing the output power of signal light issued from the optical transmitting unit by feeding the driving current changed by the driving current changing unit into the optical transmitting unit, and (g) time constant compensating unit for stopping feed of driving current into the optical transmitting unit by the driving current feed unit only at the time of optical output cut-off, compensating individual different time constants at the time of change of the transmission level and reference level in transient response by optical output cut-off or its cancellation by changing the transmission level and reference level.
That is, the power of the signal light issued from the optical transmitting unit is adjusted by feeding the changed driving current into the optical transmitting unit so that the transmission level detected by the transmission level detector depending on the reception power in part of the signal light issued by the optical transmitting unit may be equal to the reference level generated by the reference level generator on the basis of the mark rate of the transmission data. When the optical output is cut off, feed of driving current into the optical transmitting unit is stopped, and the transmission level and reference level are changed by the time constant compensating unit at the time of transient response by optical output cut-off or its cancellation, thereby compensating for the difference in change of the transmission level and reference level having mutually different time constants at the time of transient response by optical output cut-off or its cancellation.
According to this embodiment of the invention, when the optical output is cut off, feed of driving current into the optical transmitting unit is stopped, and the transmission level and reference level are changed by the time constant compensating unit at the time of transient response by optical output cut-off or its cancellation, thereby compensating for the differences in change of the transmission level and reference level having mutually different time constants at the time of transient response by optical output cut-off or its cancellation. It is therefore possible to prevent generation of excessive optical output conventionally experienced at the time of transient response by optical output cut-off or its cancellation.
The invention further relates to an optical transmitter in which the time constant compensating unit includes delay unit for stopping feed of driving current into the optical transmitting unit by the driving current feed unit only at the time of optical output cut-off, and delaying by the duration of a first time longer than the transient response time by optical output cut-off or its cancellation, and compensates individual different time constants at the time of change of the transmission level and reference level in transient response by optical output cut-off or its cancellation, for the duration of the first time after optical output cut-off or its cancellation, by changing the transmission level and reference level.
That is, when the optical output is cut off, feed of driving current is stopped, and the transmission level and reference level are changed by the time constant compensating unit by the duration of the first time longer than the transient response time after the optical output cut-off or its cancellation, thereby compensating for the difference in change of the transmission level and reference level having mutually different time constants at the time of transient response by optical output cut-off or its cancellation.
According to this embodiment of the invention, when the optical output is cut off, feed of driving current is stopped, and the transmission level and reference level are changed by the time constant compensation unit by the duration of the first time longer than the transient response time after the optical output cut-off or its cancellation, thereby compensating for the difference in change of the transmission level and reference level having mutually different time constants at the time of transient response by optical output cut-off or its cancellation. It is therefore possible to change the transmission level and reference level only by adding a simple delay circuit to the conventional optical transmitter having the APC circuit, and prevent generation of excessive optical output conventionally experienced at the time of transient response by optical output cut-off or its cancellation.
The invention also relates to an optical transmitter in which the time constant compensating unit stops feed of driving current into the optical transmitting in which the time constant compensating unit stops feed of driving current into the optical transmitting unit by driving current feed unit only at the time of optical output cut-off, and compensates individual different time constants at the time of change of the transmission level and reference level in transient response by optical output cut-off or its cancellation, so that either one of the transmission level or reference level may be always higher than the other within the transient response time by the optical output cut-off or its cancellation.
That is, when the optical output is cut off, feed of driving current is stopped, and either one of the transmission level or reference level is always set higher than the other by the time constant compensating unit at the time of transient response by the optical output cut-off or its cancellation, thereby compensating for the difference in change of the transmission level and reference level having mutually different time constants at the time of transient response by optical output cut-off or its cancellation.
According to this embodiment of the invention, when the optical output is cut off, feed of driving current is stopped, and either one of the transmission level or reference level is always is always set higher than the other by the time constant compensating unit at the time of transient response by the optical output cut-off or its cancellation, thereby compensating for the difference in change of the transmission level and reference level having mutually different time constants at the time of transient response by optical output cut-off or its cancellation. It is therefore possible to prevent generation of excessive optical output conventionally experienced at the time of transient response by optical output cut-off or its cancellation only by adding a simple element to the conventional optical transmitter having the APC circuit.
The invention additionally relates to an optical transmitter which comprises (a) optical transmitting unit for issuing a signal light by an output power corresponding to a supplied driving current, (b) optical receiving unit for receiving part of the output light from the optical transmitting unit, (c) transmission level detector for detecting a transmission level depending on the reception power by the optical receiving unit, (d) reference level generator for generating a reference level on the basis of a mark rate, (e) driving current changing unit for changing the driving current so that the transmission level detected by the transmission level detector may be equal to the reference level generated by the reference level generator, (f) driving current limit judging unit for judging if the driving current changed by the driving current changing unit has exceeded a predetermined driving current limit value or not, (g) driving current limit feed unit for feeding the driving current limit value into the optical transmitting unit when the changed driving current is judged to be over the predetermined driving current limit value by the driving current limit judging unit, (h) driving current feed unit for feeding the changed driving current into the optical transmitting unit when the changed driving current is not judged to be over the predetermined driving current limit value by the driving current limit judging unit, and (i) time constant compensating unit for stopping feed of driving current into the optical transmitting unit by the driving current feed unit only at the time of optical output cut-off, compensating individual different time constants at the time of change of the transmission level and reference level in transient response by optical output cut-off or its cancellation by changing the driving current limit value and transmission level.
That is, the power of the signal light issued from the optical transmitting unit is adjusted by feeding the changed driving current into the optical transmitting unit so that the transmission level detected by the transmission level detector depending on the reception power in part of the signal light issued by the optical transmitting unit may be equal to the reference level generated by the reference level generator on the basis of the mark rate of the transmission data. However, the driving current limit judging unit judges if the driving current supplied into the optical transmitting unit has exceeded the predetermined driving current limit value or not, and when the supplied driving current is judged to be over the driving current limit value, the driving current limit feed unit feeds the driving current at the driving current limit value into the optical transmitting unit, and when the driving current is not judged to be over the driving current limit value, the driving current is directly supplied into the optical transmitting unit. Hence, excessive driving current is not supplied into the optical transmitting unit. When the optical output cut off, feed of driving current into the optical transmitting unit is stopped, and the transmission level and driving current limit value are changed by the time constant compensating unit at the time of transient response by optical output cut-off or its cancellation, thereby compensating for the difference in change of the transmission level and reference level having mutually different time constants at the time of transient response by optical output cut-off or its cancellation. It is therefore possible to limit excessive optical output in usual operation, and prevent generation of excessive optical output at the time of transient response by optical output cut-off or its cancellation.
According to this embodiment of the invention, the driving current supplied into the optical transmitting unit is limited, and when the optical output is cut off, feed of driving current is stopped, and further the transmission level and driving current limit value are changed by the time constant compensating unit at the time of transient response by optical output cut-off or its cancellation, thereby compensating for the difference in change of the transmission level and reference level having mutually different time constants at the time of transient response optical output cut-off or its cancellation. It is therefore possible to limit excessive optical output in usual operation, and prevent generation of excessive optical output conventionally experienced at the time of transient response by optical output cut-off or its cancellation.
The invention also relates to an optical transmitter which further comprises driving current limit judging unit for judging if the driving current changed by the driving current changing unit has exceeded a predetermined driving current limit value or not, and driving current limit feed unit for feeding the driving current limit value into the optical transmitting unit when the changed driving current is judged to be over the predetermined driving current limit value by the driving current limit judging unit, in which the driving current feed unit feeds the changed driving current into the optical transmitting unit when the changed driving current is not judged to be over the predetermined driving current limit value by the driving current limit judging unit.
That is, the driving current limit judging unit judges if the driving current supplied into the optical transmitting unit has exceeded the predetermined driving current limit value or not, and when the supplied driving current is judged to be over the current limit value, the driving current limit feed unit feeds the driving current at the driving current limit value into the optical transmitting unit, and when the driving current is not judged to be over the current limit value, the driving current is directly supplied into the optical transmitting unit.
According to this embodiment of the invention, in addition to the effects of the invention of previous embodiments, since the driving current supplied into the optical transmitting unit is limited, the reliability of the optical transmitter is further enhanced.